Is it really "Collections Direct" owned by NPower chasing you, or
Egg and Collect Direct (UK) ?
It is a matter of getting to know your opponent, checking up their form before dealing with them. You might like to send a polite Private Message to the opening poster of that thread to see if he has any more info unsuited for publication in the public domain.
Egg are famous for not sending out Default Notice but claiming to have done so. It would be impossible to prove they did not. It confers no benefit on Egg or CD UK to register a default against the cardholder. They only do it to encourage settlement of arrears, or better still full payment. Have you checked with Experian whether the Default has already been put on? If so, the damage has already been done.
The danger is that when you ring or write to CD UK you are likely to be dealing with low-level low-paid staff, and the left hand does not know what undertaking the right hand previously gave the debtor. So, essential to reach a high-up manager who is hopefully more responsible, better still if you can deal with Egg. Just as you have little confidence if CD will stand by their promises, on their side they do not know if you will.
If there is some way you could convince them you will do your best to settle asap (e.g. postdated cheques?), then the other side is more likely to ease up on their frantic pressure. Letters would stand a better chance to contact the higher-ups than phone calls. Try address your letter to The Company Secretary, CD UK, marked Private and Confidential.
It is a case of trying to ensure a clear agreement of tit for tat from both sides. All they want is money really.
Good luck.